As a background technology of the present technical field, a fuel injection valve described in Japanese patent application publication No. 2005-207412 (patent document 1) has been known. In this fuel injection valve, the outer circumferences of a movable core and a fixed core are covered with a cylindrical member disposed inside a coil, and a magnetic circuit is formed by the cylindrical member, the movable core and the fixed core. The fixed core is provided with a tapered part on the opposite side to the movable core and with a large diameter part on the anti-movable core side of the tapered part. The outer diameter of the tapered part becomes large from the facing end surface side facing the movable core toward the large diameter part. The outer diameter of the facing end surface of the tapered part which faces the movable core is substantially equal to the outer diameter of the movable core. The outer diameter of the large diameter part of the fixed core is larger than that of the movable core, and the magnetic path area of the large diameter part is larger than that on the opposite side of the movable core to the fixed core (see abstract).
With this, in the fuel injection valve of the patent document 1, the magnetic path area on the anti-movable core side (large diameter part) of the fixed iron core is set to be larger than the magnetic path area on the opposite side to the fixed core of the movable core (movable iron core), and the magnetic flux quantum flowing between the movable core and the fixed core is increased, and thereby valve opening response is improved (see paragraph [0029]). In addition, by recessing the facing end surface side of the fixed core which faces the movable iron core radially inward by the tapered part, the area of the facing end surface facing the movable core becomes small, and a part of the magnetic flux is suppressed from flowing between a member covering the outer circumference of the movable core and the fixed core (see paragraph [0030]). Moreover, in the fuel injection valve of the patent document 1, the tapered part acts as a magnetic throttle, and it is possible to limit the flow of the magnetic flux between the movable core and the fixed core beyond the required quantum, and consequently, a saturated attractive force can be reduced. Therefore, the remaining magnetic flux is reduced, and thereby valve closing response is improved (see paragraph [0031]).